The invention has reference to a boiler plant comprising a combustion chamber with two or more combustion chamber spaces, one located above the other, with fluidized beds for combustion of a fuel and preferably also the removal of sulfur included in the fuel. Fuel is supplied to a first fluidized bed in a first combustion chamber space. Combustion gases and unburnt fuel particles and light ash particles, accompanying the combustion gases, flow through nozzles up into a second bed in a second combustion chamber space. The major part of the combustion takes place in the first bed. The unburnt fuel which accompanies the combustion gases to the second bed is burnt in the second bed.
The bed material in the beds normally consists of coal ash and possible quartz sand and, when desulfurization of sulfur-rich fuel is aimed at, also partially of a sulfur absorbent such as limestone or dolomite. The sulfur absorbent may be supplied to one of or all of the beds, but is preferably supplied to the second or uppermost bed. The bed material has such a granular size when being supplied to the combustion chamber that it forms beds having suitable fluidizing properties and is not blown away by the fluidizing gas.
Multibed combustion chambers comprising series arranged fluidized beds are known, for example, from U.S. Pat. No. 4,279,207.
A condition for obtaining good performance during combustion in a fluidized bed is that the temperature be maintained within rather close limits in the face of varying bed depth and changing energy outputs from the boiler plant. The lowest combustion chamber includes a nest of tubes submerged in the bed through which tubes flowing coolant removes more than half of the energy of the boiler plant. The tube nest also cools the bed material so that overheating thereof is prevented. To prevent the tube nest, under conditions of partial load, from cooling down the bed to a temperature below the temperature which is necessary for maintaining fuel combustion, the bed level is lowered so that an appropriate number of layers in the tube nest will be situated above the bed surface. The cooling effect of the tube nest on the bed material is then reduced.
In known multibed combustion chambers, to control depth of bed material, bed material is discharged from the lower bed (where the greater part of the combustion takes place). Discharged bed material is stored during partial load operation in a storage space located outside the actual combustion chamber. According to U.S. Pat. No. 4,279,207, material discharged from a bed is stored in a space below the combustion chamber space containing the combustion bed. The bed material in this storage space is maintained in fluidized condition by the combustion air and will thus be cooled by the air. When the bed material comes to be fed back into the combustion bed upon an increase in power demand, it is cooled bed material that will be fed back. The necessary reheating of the returned bed material delays the rate of power increase possible in the plant, so that this rate is lower than it would be were the bed material to be returned at substantially the same temperature as that existing in the bed.